Index: kernel/arch/amd64/include/proc/thread.h
===================================================================
--- kernel/arch/amd64/include/proc/thread.h	(revision c03ee1c304c982b593d8043411e6b003dd1c155a)
+++ kernel/arch/amd64/include/proc/thread.h	(revision 9cc0d7c4f78aa4efea62a2d761476193f7e7717d)
@@ -36,6 +36,12 @@
 #define KERN_amd64_THREAD_H_
 
+/* CAUTION: keep these in sync with low level assembly code in syscall_entry */
+#define SYSCALL_USTACK_RSP	0
+#define SYSCALL_KSTACK_RSP	1
+
 typedef struct {
 	unative_t tls;
+	/** User and kernel RSP for syscalls. */
+	uint64_t syscall_rsp[2];	
 } thread_arch_t;